This invention relates to reinforcing sheets such as those adapted for applying localized reinforcement to sheet metal or sheet plastic structures.
It is common practice in the automotive industry to apply reinforcing sheets to sheet metal and other parts for localized, lightweight reinforcement. Examples of such reinforcing sheets are described, for example, in U.S. Pat. No. 4,444,818 to Tominaga, U.S. Pat. No. 4,766,183 and U.S. Pat. No. 4,842,938 to Rizk et al., U.S. Pat. No. 4,803,105 to Kretow et al., U.S. Pat. No. 4,803,108 to Leuchten et al., and U.S. Pat. Nos. 4,900,601, 4,929,483 and 5,092,947 to Hxc3xa4lg et al. Generally, these reinforcing sheets include one or more layers of a stiffening material and one or more layers of a polymeric material that acts as a binder for the stiffening material as well as an adhesive for securing the reinforcing sheet to a substrate. Often, protective foils, moisture barriers and other layers may be included in the reinforcing sheet.
A common adhesive for these reinforcing sheets is described in U.S. Pat. No. 4,803,105. That adhesive includes a mixture of an epoxy resin, a curing agent and a carboxy-terminated butadiene-acrylonitrile rubber. The butadiene-acrylonitrile rubber commonly used contains about 18 weight % or more polymerized acrylonitrile, has a glass transition temperature of about xe2x88x9252xc2x0 C. or higher and has a solubility parameter, as reported by its manufacturer, of about 8.82 or above. The amount of the butadiene-acrylonitrile rubber is such that the adhesive as a whole contains about 4% or more by weight polymerized acrylonitrile. The rubber is typically pre-reacted with a portion of the epoxy resin before being formulated into the adhesive mixture. The rubber imparts a certain amount of toughness to the adhesive once it is cured.
In the automotive industry, these reinforcing sheets are typically applied to exterior body panels. Because the epoxy resin in these reinforcing sheets must be cured, the reinforcing sheets are usually applied before the body panel is painted, so that the epoxy resin and paint can be cured simultaneously. Often, the unpainted body panel (or other substrate) is contaminated with oily materials. In other cases, the substrate is cold for one reason or another when the reinforcing sheet is applied.
The epoxy adhesive commonly used in the reinforcing sheets does not adhere well to oily or cold surfaces. When these conditions are present, the manufacturer either accepts that a certain amount of reinforcing sheets will be poorly adhered, or else the substrate parts must be warmed and/or cleaned before applying the reinforcing sheet. Neither of these options is attractive. Warming and cleaning the substrate introduce extra steps into the process of assembling a vehicle. This in turn imposes extra costs for time, energy and handling. It would be desirable to provide a reinforcing sheet that adheres better to oily or cold substrates.
This invention is a reinforcing sheet comprising at least one layer of a reinforcing material and a layer of a solid thermosetting adhesive, wherein the thermosetting adhesive includes (a) a curing agent and (b) an epoxy-terminated adduct of an epoxy resin and a conjugated diene or conjugated diene/nitrile rubber containing not more than about 15% by weight polymerized nitrile monomer, and wherein said thermosetting adhesive contains no more than about 3.5% by weight polymerized nitrile monomer.
Surprisingly, the reinforcing sheet of the invention adheres well to oily substrates as well as to colder substrates, compared to otherwise similar reinforcing sheets in which the nitrile monomer content in the thermosetting adhesive layer is somewhat higher.
In a second aspect, this invention is a method of reinforcing a substrate, comprising
(A) applying to the substrate a reinforcing sheet comprising at least one layer of a reinforcing material and a layer of a solid thermosetting adhesive, wherein the thermosetting adhesive includes (a) a curing agent and (b) an epoxy-terminated adduct of an epoxy resin and a conjugated diene or conjugated diene/nitrile rubber containing not more than about 15% by weight polymerized nitrile monomer, and wherein said thermosetting adhesive contains no more than about 3.5% by weight polymerized nitrile monomer, and then
(B) curing said thermosetting adhesive.
The thermosetting adhesive layer of the invention includes an epoxy-terminated adduct of an epoxy resin and a diene rubber or a conjugated diene/nitrile rubber having containing not more than about 15% by weight polymerized nitrile monomer. This adduct is suitably prepared in the reaction of a polyepoxide with a carboxy-functional conjugated diene rubber or a conjugated diene/nitrile rubber.
The diene rubber is a polymer of a conjugated diene monomer such as butadiene, isoprene, and the like. Butadiene rubbers are preferred. Conjugated diene/nitrile rubbers are copolymers of a conjugated diene and an ethylenically unsaturated nitrile monomer, of which acrylonitrile is the most preferred one. When a conjugated diene/nitrile rubber is used, at least one such rubber present in the composition contains less than about 15 weight percent polymerized unsaturated nitrile, and preferably no more than about 12 weight percent polymerized unsaturated nitrile. The rubber also contains terminal groups that will react with an epoxide to form a covalent bond thereto. Preferably, the rubber contains from about 1.5, more preferably from about 1.8, to about 2.5, more preferably to about 2.2, of such terminal groups per molecule, on average. Carboxyl-terminated rubbers are preferred.
The rubber is preferably a liquid at room temperature, and preferably has a glass transition temperature of less than about xe2x88x9255xc2x0 C., preferably from about xe2x88x9260 to about xe2x88x9290xc2x0 C. The molecular weight (Mn) of the rubber is suitably about 2000 to about 6000, more preferably from about 3000 to about 5000.
Suitable carboxyl-functional butadiene and butadiene/acrylonitrile rubbers are commercially available from B. F. Goodrich under the trade names Hycar(copyright) 2000X162 carboxyl-terminated butadiene homopolymer and Hycar(copyright) 1300X31 carboxyl-terminated butadiene/acrylonitrile copolymer. A suitable amine-terminated butadiene/acrylonitrile copolymer is sold under the tradename Hycar(copyright) 1300X21.
In addition, a conjugated diene or conjugated diene/nitrile rubber having a somewhat higher polymerized nitrile content (xe2x80x9chigh nitrile rubberxe2x80x9d) can be used in addition to the rubber described above. Higher acrylonitrile content provides better adhesion between the polymer and some substrates such as glass, and tends to increase flexural strength somewhat. The high nitrile rubber also preferably contains terminal epoxy-reactive groups. The high nitrile rubber advantageously contains from above 15%, more preferably at least about 18% to about 40%, more preferably to about 32% polymerized nitrile monomer. The high nitrile rubber suitably has a molecular weight of about 3000 to about 6000, and contains from about 1.5 more preferably from about 1.8, to about 2.5, more preferably to about 2.2, epoxy-reactive terminal groups per molecule, on average. Examples of such high nitrile rubbers are Hycar(copyright) 1300X8, Hycar(copyright) 1300X13(copyright), Hycar(copyright) 1300X9 and Hycar(copyright) 1300X18 carboxyl-terminated butadiene acrylonitrile copolymers, all commercially available from B. F. Goodrich.
The rubbers (i.e., the conjugated diene or conjugated diene/nitrile rubber, plus the high nitrile rubber, if used) are present in an amount such that the thermosetting composition contains no more than about 3.5% polymerized nitrile monomer. It is more preferred that the thermosetting adhesive contains from about 1 to about 3.25% by weight polymerized nitrile monomer. Preferably, the rubbers constitute from about 5, more preferably from about 10, to about 40, more preferably to about 25% of the total weight of the thermosetting adhesive. The high nitrile rubber preferably constitutes no more than 50, preferably no more than about 45%, more preferably no more than about 35% of the combined weight of all rubbers.
The conjugated diene or conjugated diene/nitrile rubber is formed into an epoxy-terminated adduct by reaction with an excess of a polyepoxide. Any high nitrile rubber that is used is similarly converted into an epoxy-terminated adduct. A wide variety of polyepoxide compounds such as cycloaliphatic epoxides, epoxidized novolac resins, epoxidized bisphenol A or bisphenol F resins, butanediol polyglycidyl ether, neopentyl glycol polyglycidyl ether or flexibilizing epoxy resins can be used, but generally preferred on the basis of cost and availability are liquid or solid glycidyl ethers of a bisphenol such as bisphenol A or bisphenol F. Halogenated, particularly brominated, resins can be used to impart flame retardant properties if desired. For forming the adduct, liquid epoxy resins (such as DER 331, available from The Dow Chemical Company) are especially preferred for ease of handling in making the adduct. Typically, the rubber and an excess of the polyepoxide are mixed together with a polymerization catalyst such as a substituted urea or phosphine catalyst, and heated to a temperature of about 100-250xc2x0 C. in order to form the adduct. Preferred catalysts include phenyl dimethyl urea and triphenyl phosphine. Preferably, enough of the polyepoxide compound is used that the resulting product is a mixture of the adduct and free polyepoxide compound.
If the adduct is not a solid at room temperature (about 28xc2x0 F.), it is suitably blended with a normally solid (at room temperature) epoxy resin. An example of such a solid epoxy resin is a polyglycidyl ether of bisphenol A or bisphenol F having an epoxy equivalent weight of about 300 to about 1000. Enough of the solid epoxy resin is blended with the adduct so the resulting mixture is solid at room temperature.
Alternatively, blends of two or more polyepoxide compounds can be reacted with the rubber so as to directly form a solid adduct.
The thermosetting adhesive also contains a curing agent. A large number of curing agents are useful, particularly those that require elevated temperatures (i.e., above about 50xc2x0 C.) to cure. Advantageously, Lewis acids, substituted imidazoles or amine salts can be used as curing agents. Blocked amine curing agents such as those made by the reaction of approximately equimolar amounts of an anhydride and a polyamine are also useful. Such blocked amine curing agents are described in U.S. Pat. No. 4,766,183, the relevant portions of which are incorporated by reference. An especially useful curing agent is dicyandiamide. The curing agent is used in amounts sufficient to provide a complete cure, such as about 0.25 to about 10, preferably about 2 to about 5 percent of the weight of the thermosetting adhesive.
The thermosetting adhesive preferably is cellular when fully cured and/or contains a quantity of microspheres in order to reduce density. Suitable microspheres include those made from inorganic materials such as glass and silica-alumina ceramics or polymeric materials such as epoxy resin, unsaturated polyester resin, silicone resin, phenolics, polyvinyl alcohol, polyvinyl chloride, polypropylene, and polystyrene. In addition, fly ash that is in the form of hollow particles can be used. Examples of commercially available fly ash of this type is sold by Boliden Intertrade, Inc., under the trade names Fillite 100 and Fillite 150. Glass microspheres are most preferred. These microspheres most advantageously have average diameters of from about 5 to about 150 microns, preferably from about 20 to about 85 microns. In addition, the microspheres advantageously have a bulk density of from about 0.1 to about 0.5 g/cc. If desired, the microspheres may be surface treated with an interfacial adhesion promoter such as a silane compound.
Microspheres preferably constitute from about 5, more preferably from about 10, to about 30, more preferably to about 20, percent of the total weight of the thermosetting adhesive.
In addition, the thermosetting adhesive may also contain a blowing agent that becomes activated at the curing temperature to expand the adhesive layer. Suitable blowing agents include physical blowing agents such as hydrocarbons, hydrofluorocarbons, hydrochlorofluorocarbons and the like, but more preferably include chemical blowing agents such as azobisformamide and the like.
In addition, the thermosetting adhesive may contain one or more other fillers or rheology control agents. Suitable fillers include talcs, clays, silicas, calcium carbonate, graphite, glass, carbon black, plastic powders such as ABS, and the like. Magnetic particles such as ferromagnetic particles may be used as a filler, as well. Suitable rheology control agents include fumed silica and certain clays such as bentonite clay and montmorillonite clay.
The thermosetting adhesive can include a plasticizer for impact and thermal shock resistance improvement. Advantageously, benzoates, adipates, terephthalates and phthalates can be used as the plasticizer. A terephthalate or phthalate, for example dibutyl phthalate, is preferred.
In addition, the thermosetting adhesive can further contain a flame retardant, such as hydrated alumina or antimony oxide.
The thermosetting adhesive is preferably formulated to be tacky at a temperature of about 15xc2x0 to about 40xc2x0 C.
The thermosetting adhesive is applied to at least one side of a reinforcing layer. The reinforcing layer is preferably made of a stiff yet flexible construction, in order to provide reinforcement when applied to a substrate, and yet conform to the shape of the substrate. Preferred reinforcing materials are fibers of stiff materials such as glass, polyamide resin, polypropylene resin, carbon and the like, as well as aluminum sheet or foil, films of high melting thermoplastic resins such as Mylar, that may be fiber-reinforced. More preferred reinforcing materials are woven fabrics of stiff fibers as just described, especially woven glass fabrics. Mixtures of two or more different fibers can be woven together if desired. For example, carbon fibers may be woven into a glass fabric to increase stiffness at a moderate cost. The reinforcing layer preferably has a thickness of 0.003 inch to 0.050 inch.
The reinforcing layer may have a planar and/or smooth configuration, or may include three-dimensional features to further increase stiffness and/or adapt the reinforcing sheet for a particular application. For example, the reinforcing layer may have a ribbed configuration such as is described in FIGS. 3-5 of U.S. Pat. No. 4,803,105, the relevant portions of which are incorporated herein by reference.
Another suitable reinforcing layer is a honeycomb structure as described in U.S. Pat. No. 4,803,108. These honeycomb structures include a perforated honeycomb member having columns that define cell apertures with open ends. The honeycomb structure is suitably formed of any material that remains stable up to the curing temperature of the thermosetting adhesive and exhibits sufficient adhesion to the thermosetting adhesive layer and sufficient flexibility to conform to the shape of the panel to be reinforced. Advantageously, the honeycomb structure is formed of a metal alloy plate. Because of its light weight, corrosion resistance, ready accessibility, inexpensive cost and high flexibility, aluminum is most preferred for forming the honeycomb structure.
The columns of the honeycomb structure are integrally connected to form a multitude of cell apertures with open ends. The cells of the honeycomb structure may be hexagonal, triangular, square, polyhedral or other convenient shapes. The columns of the honeycomb structure are sufficiently thick, and the cell apertures defined by the columns have a suitable cell size and core density, such that the honeycomb structure maintains its integrity while maintaining the capability to conform to the shape of the substrate and without unacceptably increasing the weight of the reinforcing sheet. The preferred aluminum honeycomb columns suitably have a thickness of 0.0005 inch to 0.005 inch (0.013-0.13 mm, preferably 0.0007 inch to 0.004 inch (0.0018-0.1 mm). The cell apertures suitably have a cell size of {fraction (1/16)} inch to xe2x85x9e inch (1.6-22.2 mm), preferably {fraction (3/16)} inch to {fraction (5/16)} inch (4.8-8.0 mm), and a core de of 1.0 pound per cubic foot to 12.0 pounds per cubic foot (1.6-19.2 kg/m3), preferably 3.0 pounds per cubic foot to 8.0 pounds per cubic foot (4.8-12.8 kg/m3). The honeycomb member suitably has a thickness of {fraction (1/16)} inch to 4 inches (1.6-102 mm), preferably xe2x85x9 inch to xc2xe inch (3.2-19 mm).
In addition, the reinforcing sheet may contain one or more additional functional layers, such as a moisture barrier layer as described in U.S. Pat. No. 4,803,105. The presence of such a moisture impermeable barrier permits the reinforcing sheet to be stored for long periods of time, for example three to six months, with minimal adverse effects due to the absorption of moisture, even under humid conditions. Another useful functional layer is a release sheet, typically paper, which covers the exposed surface of the adhesive layer that is brought into contact with the substrate to be reinforced. If desired, a slit, heat-shrinkable protection foil of the type described in U.S. Pat. No. 4,900,601 can be used on the surface of the adhesive layer.
The reinforcing sheet advantageously is prepared by applying a layer of the thermosetting adhesive to the reinforcing layer. A convenient way of accomplishing this is to spread a layer of the thermosetting adhesive onto a release layer or protection foil with a coating knife to a uniform thickness suitably of 0.01 inch to 0.10 inch (0.25-2.5 mm), preferably 0.02 inch to 0.05 inch (0.5-1.25 mm). The reinforcing layer is then placed on the thermosetting adhesive layer and pressed in with a pressure roll. The entire reinforcing sheet is then pressed with a roller to provide a sheet with total thickness suitably of 0.03 inch to 0.30 inch (0.75-7.5 mm), preferably 0.04 inch to 0.10 inch (1.0-2.5 mm).
To apply the reinforcing sheet to the substrate to be reinforced, the exposed surface of the thermosetting adhesive layer is brought in contact with the panel. An advantage of the reinforcing sheet of this invention is that it adheres well to substrates that are somewhat oily or somewhat cold. Thus, the reinforcing sheet is of particular interest when applied to a substrate that is at a temperature of from about 15xc2x0 C. to about 40xc2x0 C., preferably from about 12xc2x0 C. to about 35xc2x0 C. The reinforcing sheet of the invention exhibits substantially improved adhesion to substrates at temperatures of about 15xc2x0 C. to about 22xc2x0 C., compared to previous reinforcing sheets in which the thermosetting adhesive contained only a high nitrile rubber.
The substrate and applied reinforcing sheet are subsequently heated to cure the thermosetting adhesive. This is conveniently done at a temperature of from about 150xc2x0 C. to about 200xc2x0 C., for a period of about 15 minutes to about 1 hour. This curing step can be done simultaneously with other treatments requiring heating, such as curing paints or E-coats.
The following examples are provided to illustrate the invention, but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.